4. The problem of ageing steels

4.1 Natural and artificial aging

Mild and extra-mild steels can exhibit the phenomenon of ageing, even in the absence of intentional additions of alloying elements. This phenomenon occurs when the metal is cooled at varying rates after being heated to temperatures of 600 to 700°C, especially if it has been work-hardened. Aging is characterized by spontaneous changes in the mechanical properties of steel over time.

These changes include :

• an increase in strength characteristics, such as hardness ;

  • yield point,

  • breaking load ;

• reduced ductility characteristics, such as :

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